The toner for forming electrostatic images generally comprises, as the toner mother particles, fine particles of a binder resin containing a coloring component, e.g., a dye or a pigment, and, if necessary, a charge controlling agent, and the toner is obtained by a method of adding external additives to the outside (surface) of the toner mother particles for the purpose of providing flowability or controlling an electrification property. As the external additives, positively electrifiable silica fine particles, negatively electrifiable silica fine particles, inorganic fine particles other than silica (e.g., titanium oxide), fatty acid metal salt and the like are used.
In general, the toner for forming electrostatic images is negatively charged. Such a toner is obtained by preparing negatively electrifiable toner mother particles and adding external additives, e.g., positively electrifiable silica fine particles, etc., to the negatively electrifiable toner mother particles, to thereby control the quantity of negative electrification (refer to, e.g., patent documents 1 to 3 set forth below). Alternatively, when from weakly negatively electrifiable toner mother particles to positively electrifiable toner mother particles are used, there is a method of adding external additives, such as negatively electrifiable silica fine particles, etc., to the above toner mother particles to control the quantity of negative electrification (refer to, e.g., patent documents 4 to 6).
As the methods of manufacturing a toner by using negatively electrifiable toner mother particles, a method of externally adding positively electrifiable hydrophobic silica fine particles to toner mother particles comprising a negatively electrifiable binder resin, a method of externally adding positively electrifiable hydrophobic silica fine particles and negatively electrifiable hydrophobic silica fine particles (refer to, e.g., patent documents 1 and 2), and a method of externally adding positively electrifiable hydrophobic silica fine particles and inorganic fine particles having a low electrical resistance value (refer to, e.g., patent document 3) are exemplified.
On the other hand, when weakly negatively electrifiable to positively electrifiable toner mother particles are used in manufacturing a toner, external addition methods are also examined. For example, a method of externally adding positively electrifiable hydrophobic silica fine particles and negatively electrifiable hydrophobic silica fine particles at the same time (refer to, e.g., patent document 4), and a method of externally adding a first component, a second component, a third component and a fourth component to toner mother particles at the same time, or externally adding the first component lastly, taking hydrophobic silica fine particles or hydrophobic titania as the first component, hydrophobic silica fine particles or hydrophobic titania having larger particle sizes than the particle sizes of component 1 as the second component, inorganic fine particles as the third component, and a fatty acid metal salt as the fourth component (refer to, e.g., patent document 5) are known.
Further, there is disclosed in a patent document a method to obtain a toner in which the isolation of external additives is restrained by externally adding in the order of titanium oxide fine particles and silica fine particles to toner mother particles (refer to, e.g., patent document 6).
However, in the toners obtained by the methods disclosed in patent documents 1 to 6, external additives (positively electrifiable silica fine particles, negatively electrifiable silica fine particles, titanium oxide fine particles and the like), which function to control electrification or flowability, are liable to be desorbed from the surface of the toner, which causes the reduction of flowability or electrification property of the toners, as a result, transfer efficiency and image density are depressed.
In addition, it is disclosed in patent documents 1 and 5 to use the metal salt of a fatty acid (a metal soap) in the toner for electrophotography, and the cases of using the metal salt of a fatty acid besides the above are also disclosed in patent documents 7 to 12. In patent document 5, a fatty acid metal salt is used in view of the prevention of fixing of a toner and generation of black spots on the surface of a photosensitive material, and then hydrophobic silica or hydrophobic titania is added. However, there is a problem that the external additive hydrophobic silica or hydrophobic titania is liberated, so that the stability of electrification cannot be maintained for a long period of time.
Patent document 7 perceived the binding property of a fatty acid metal salt and obtained a toner by the external addition step of only one time of the addition of a fatty acid metal salt with silica having amino groups on the surface at the same time. Although the toner obtained by this method is improved a little in the point of the rate of isolation of hydrophobic silica, a problem that the quantity of electrification of the toner cannot be maintained stably is still left behind. Patent documents 8 to 12 also disclose the use of fatty acid metal salts as the external additive in manufacturing toners. However, in any of the above patent document similarly to patent document 7, the fatty acid metal salts are added with other external additives at the same stage to obtain toners by the external addition process of only one time. The toners disclosed in patent documents 7 to 12 also have problems that the quantity of electrification of the toners cannot be maintained stably.
[Patent document 1]
JP-A-2000-267337 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
[Patent document 2]
JP-A-2002-14487
[Patent document 3]
JP-A-2002-214834
[Patent document 4]
JP-A-11-231571
[Patent document 5]
JP-A-2001-100452
[Patent document 6]
JP-A-2002-72544
[Patent document 7]
Japanese Patent 2502353
[Patent document 8]
JP-B-8-33681 (the term “JP-B” as used herein means an “examined Japanese patent publication”).
[Patent document 9]
Japanese Patent 2759510
[Patent document 10]
JP-A-9-114129
[Patent document 11]
JP-A-11-323396
[Patent document 12]
JP-A-2001-296688
[Patent document 13]
JP-A-2002-202622
An object of the present invention is to provide a toner which is low in desorption of external additives (e.g., positively electrifiable silica fine particles, negatively electrifiable silica fine particles, titanium oxide fine particles and the like), can maintain the electrification property for a long period of time, shows high flowability and transfer efficiency, and is not accompanied by the reduction of image density. Additionally, an object of the present invention is also to provide an image-forming apparatus using the toner.